1. Field of the Invention
The present invention relates to an apparatus and a method, both for controlling a spatial light modulator (SLM) comprised in a video image display apparatus. More particularly, the present invention relates to a video image display apparatus implemented with multiple spatial light modulators for reflecting and modulating light of different colors controlled to project the lights with specially arranged time sequences.
2. Description of the Related Art
Even though there have been significant advances made in recent years in the technology of implementing electromechanical micromirror devices as spatial light modulators (SLM), there are still limitations and difficulties when these are employed to display high quality images. Specifically, when the display images are digitally controlled, the quality of the images is adversely affected because the images are not displayed with a sufficient number of gray scale gradations.
Electromechanical mirror devices are drawing a considerable amount of interest as spatial light modulators (SLM). The electromechanical mirror device consists of a mirror array arranging a large number of mirror elements. In general, the number of mirror elements range from 60,000 to several millions and are arranged on the surface of a substrate in an electromechanical mirror device.
Refer to FIG. 1A for a digital video system 1 as disclosed in a relevant U.S. Pat. No. 5,214,420, which includes a display screen 2. A light source 10 is used to generate light energy to illuminate display screen 2. Light 9 is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13, and 14 serve a combined function as a beam collimator to direct light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. The SLM 15 has a surface 16 that includes switchable reflective elements, e.g., micro-mirror devices 32 with elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30, as shown in FIG. 1B. When element 17 is in one position, a portion of the light from path 7 is redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, light is not redirected toward display screen 2 and hence pixel 3 would be dark.
Each of the mirror elements constituting a mirror device functions as a spatial light modulator (SLM), and each mirror element comprises a mirror and electrodes. A voltage applied to the electrode(s) generates a Coulomb force between the mirror and the electrode(s), making it possible to control and incline the mirror. The inclined mirror is “deflected” according to a common term used in this patent application for describing the operational condition of a mirror element.
When a mirror is deflected with a voltage applied to the electrode(s), the deflected mirror also changes the direction of the reflected light in reflecting an incident light. The direction of the reflected light is changed in accordance with the deflection angle of the mirror. The present patent application refers to the light reflected to a projection path designated for image display as “ON light”, and refers to a light reflected in a direction away from the designated projection path for image display as “OFF light”. When only a portion of the reflected light is directed in the ON light direction and the light reflected by the mirror to the projection path is of lesser intensity than the “ON light”, it is referred to as “intermediate light”.
The present patent application defines an angle of rotation along a clockwise (CW) direction as a positive (+) angle and that of a counterclockwise (CCW) direction as a negative (−) angle. A deflection angle is defined as zero degrees (0°) when the mirror is in the initial state.
The on-and-off states of a micromirror control scheme, such as that implemented in the U.S. Pat. No. 5,214,420 and by most conventional display systems, limit image display quality. This is because the application of a conventional control circuit limits the gray scale (PWM between ON and OFF states) by the LSB (least significant bit, or the least pulse width). Due to the ON-OFF states implemented in conventional systems, there is no way to provide a pulse width shorter than the LSB. The least brightness, which determines the gray scale, is the light reflected during the least pulse width. A limited gray scale leads to lower image quality.
In FIG. 1C, a circuit diagram of a control circuit for a micro-mirror according to U.S. Pat. No. 5,285,407 is presented. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where * designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors, M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads presented to memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the Static Random Access switch Memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a word-line. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The mirror is driven by a voltage applied to the landing electrode and is held at a predetermined deflection angle on the landing electrode. An elastic “landing chip” is formed on the portion of the landing electrode that comes into contact with the mirror, and assists in deflecting the mirror towards the opposite direction when the deflection of the mirror is switched. The landing chip is designed to have the same potential as the landing electrode so that a shorting is prevented when the landing electrode is in contact with the mirror.
Each mirror formed on a device substrate has a square or rectangular shape, and each side has a length of 4 to 15 um. In this configuration, a portion of the reflected light is reflected not from the mirror surface but from the gaps between the mirrors or other surfaces of the mirror device. These “unintentional” reflections are not applied to project an image and are inadvertently generated. The contrast of the displayed image is degraded due to the interference from these unintentional reflections generated by the gaps between the mirrors. In order to overcome this problem, the mirrors are arranged on a semiconductor wafer substrate with a layout to minimize the gaps between the mirrors. One mirror device is generally designed to include an appropriate number of mirror elements, wherein each mirror element is manufactured as a deflectable mirror on the substrate for displaying a pixel of an image. The appropriate number of elements for displaying an image is configured in compliance with the display resolution standard according to the VESA Standard defined by the Video Electronics Standards Association or by television broadcast standards. When a mirror device is configured with the number of mirror elements in compliance with WXGA (resolution: 1280 by 768) defined by VESA, the pitch between the mirrors of the mirror device is 10 μm, and the diagonal length of the mirror array is about 0.6 inches.
The control circuit, as illustrated in FIG. 1C, controls the mirrors to switch between two states, and the control circuit drives the mirror to oscillate to either an ON or OFF deflected angle (or position) as shown in FIG. 1A.
The minimum intensity of light reflected from each mirror element for image display, i.e., the resolution of gray scale of image display for a digitally-controlled image display apparatus, is determined by the least length of time that the mirror may be controlled to stay in the ON position. The length of time a micromirror is in an ON position is controlled by a multiple bit word. FIG. 1D shows the “binary time intervals” when controlling micromirrors with a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8, which in turn define the relative brightness for each of the four bits, where “1” is the least significant bit and “8” is the most significant bit. According to the control mechanism as shown, the minimum controllable differences between gray scales for showing different levels of brightness is a represented by the “least significant bit” that maintains the micromirror at an ON position.
For example, assuming n bits of gray scales, one time frame is divided into 2n−1 equal time periods. For a 16.7-millisecond frame period and n-bit intensity values, the time period is 16.7/(2n−1) milliseconds.
Among conventional display apparatuses, comprised of one SLM as described above, a color display is projected by changing over, in a time sequence, the colors of light to be displayed onto a screen using a wheel, which is known as a color wheel and which comprises a plurality of color filters (e.g., red (R), green (G) and blue (B) color filters) possessing different wavelength bands of transmission light in a plurality of regions, and a plurality of laser lights (e.g., R, G and B laser lights) that emit the lights of different wavelength bands.
Such a display apparatus, however, attains a color display by projecting each of a plurality of color lights in a time sequence, and therefore a distortion phenomenon known as a “color breakup” (or a rainbow effect) is known to occur. A color breakup occurs when a rainbow-like image is instantly visible when, for example, a viewer shifts his or her point of focus on the screen.
Accordingly, it is desirable to design a display apparatus such that a color display is projected through the projection of a plurality of color lights onto a screen in a time sequence, while suppressing the occurrence of the above described color breakup.